1. Field of the Invention
The present invention relates to an AM modulated wave eliminating circuit which extracts a digital modulated wave by eliminating an AM modulated wave from an AM data multiplex modulated wave consisting of an AM stereo modulated wave multiplexed with a digital modulated wave.
2. Related Background Art
Japanese Laid-Open Patent Application No. 9-326836 discloses an AM data multiplex modulation system which multiplexes an AM modulated wave with a digital signal so as not to influence an AM synchronous detection wave output at AM synchronous detection.
The AM data multiplex modulation system mentioned above will be described with reference to FIG. 6 taking an example wherein modulation system uses the QPSK (four-phase PSK) modulation mode for a digital wave.
An analog signal wave such as a voice signal (hereinafter also referred to simply as a signal) is supplied to an AM modulator 31 and a carrier wave having a frequency fc is subjected to AM modulation with the signal wave. An AM modulated wave VAMt) output from the AM modulator 31 is expressed by the following Equation (1): EQU .nu.AM(t)={1+.kappa..nu.m(t)}cos .omega.ct (1)
wherein an amplitude of the carrier wave is taken as 1, the reference symbol .omega.c(rad/s) represents an angular frequency of the carrier wave, the reference symbol .kappa. designates a modulation degree and a reference symbol .nu.m(t) denotes the signal wave.
Arrays of digital signals of I and Q which are generated by a QPSK baseband digital signal generator 32 will be represented as In and Qn. Let us assume that In=.+-.1 and Qn=.+-.1.
The output signal from the QPSK baseband digital signal generator 32 is split into two, one being input into a quadrature modulator 33 to which a carrier wave having a frequency (fc+f.alpha. is supplied for DC-AC modulation of a carrier wave having an angular frequency (.omega.c+.omega..alpha.)(rad/s) with a complex signal array. An output signal .nu.DH(t) from the quadrature modulator 33 is as expressed by the following Equation (2): EQU .nu.DH(t)=In cos(.omega.c+.omega..alpha.t+Qn sin(.omega.c+.omega..alpha.)t (2)
On the other hand, the QPSK baseband digital signals which are output from the QPSK baseband digital signal generator 32 are supplied to a sign inverter 34 for conversion into (-In) and (-Qn). The QPSK baseband digital signals which are subjected to the signal inversion by the sign inverter 34 are supplied to a complex conjugater 35 and made conjugate in complex, whereby the sign of the Qn signal array of the QPSK baseband digital signals is inverted, whereby the signal arrays are converted into (-In) and (Qn). That is, the sign inverter 34 and the complex conjugater 35 invert a sign of components which have a vector deviation corresponding to dibits formed by the QPSK baseband digital signals and the same phase as that of a standard carrier wave.
The complex signal arrays which are made conjugate in complex by the complex conjugater 35 are input into a quadrature modulator 36 to which the carrier wave having the frequency (fc-f.alpha.) is supplied and the carrier wave having the angular frequency (fc-f.alpha.) (rad/s) is subjected to quadrature conversion with the complex signal array. An output signal .nu.DL(t) from the quadrature modulator 36 is as expressed by the following Equation (3): EQU .nu.DL(t)=-In cos(.omega.c-.omega..alpha.)t+Qn sin(.omega.c-.omega..alpha.)t (3)
The output signals .nu.DH(t) and .nu.DL(t) expressed by the Equations (2) and (3) are added to each other with an adder 37, which provides an addition output of .nu.D(t) expressed by the following Equation (4): EQU .nu.D(t)=.nu.DH(t)+.nu.DL(t)
=In cos(.omega.c+.omega..alpha.)t+Qn sin(.omega.c+.omega..alpha.)t EQU -In cos(.omega.c-.omega..alpha.)t+Qn sin(.omega.c-.omega..alpha.)t (4)
The AM modulated wave .nu.AM(t) and the digital modulated wave .nu.D(t) are input for addition into an adder 38, which transforms the modulated waves expressed by the Equations (1) and (4) into an AM data multiplex modulated wave .nu.(t) expressed by the following Equation (5): EQU .nu.(t)=.nu.AM(t)+.nu.D(t) EQU ={1+.kappa..nu.m(t)}cos .omega.ct EQU +In cos(.omega.c+.omega..alpha.)t+Qn sin(.omega.c+.omega..alpha.)t EQU -In cos(.omega.c-.omega..alpha.)t+Qn sin(.omega.-.omega..alpha.)t (5)
A process to prepare the AM data multiplex modulation wave in the AM data multiplex wave modulation system is shown in FIG. 7, wherein the AM modulated wave output from the AM modulator 31 is indicated by a, the output signal from the quadrature modulator 36, that is, the digital modulated wave, is indicated by b and the output signal from the quadrature modulator 33, that is, the digital modulated wave is indicated by c. The digital modulated wave output from the adder 37 is a sum of the signals indicated by b and c in FIG. 7, and the AM multiplex modulated wave output from the adder 38 is indicated by d in FIG. 7.
The AM data multiplex wave modulation system does not influence an AM synchronous detection wave output at an AM synchronous detection of the AM data multiplex modulated wave since the digital modulated wave signals are multiplexed at a location of the frequency (fc+f.alpha.) and a location of the frequency (fc-f.alpha.) which are axially symmetrical with regard to the carrier wave fc on a frequency axis.
However, the AM data multiplex modulation system may adopt the AM stereo modulation mode in place of the digital modulation mode for the AM data multiplex modulation. When the digital modulation mode is replaced with the AM stereo modulator, the AM data multiplex modulation system is incapable of extracting a digital modulated wave since a phase modulated wave and a digital modulated wave have similar characteristics in an AM stereo modulated wave.
Furthermore, the modulation system described above does not permit taking out desired digital data at an optionally selected time at which data are multiplexed or at an optionally selected frequency band at which data are multiplexed since it multiplexes, at the same frequency band and at the same time, an AM modulated component and a data modulated component of an AM data multiplex modulated wave which is multiplexed with a digital modulated wave and modulated in the AM data multiplex modulation mode though the system modulates an amplitude of a carrier wave having a frequency fc by an analog signal wave with an AM modulator, and multiplexes the digital modulated signals at the location of the frequency (fc+f.alpha.) and the location of the frequency (fc-f.alpha.) which are axially symmetrical with regard to the carrier wave having the frequency fc on the frequency axis.
A primary object of the present invention is to provide an AM modulated wave eliminating circuit which is capable of extracting a digital modulated wave by cancelling an AM modulated wave from an AM data multiplex modulated wave.
The AM modulated wave eliminating circuit according to the present invention extracts a digital modulated wave by cancelling an AM modulated wave from an AM data multiplex modulated wave which is multiplexed with an AM stereo modulated wave and a digital modulated wave within the same frequency band of an AM stereo modulated wave, the AM modulated wave eliminating circuit comprising extracting means to extract a composite wave composed of an AM carrier wave in-phase signal, an AM carrier wave reverse-phase signal and a digital modulated wave from an AM data multiplex modulated wave, presuming means to presume a value on the basis of a phase modulated component of an AM stereo modulated wave from the composite wave, and operating means to multiply a presumed value on the basis of the phase modulated component by the AM carrier wave in-phase component and add the multiplication output to the composite wave, characterized in that an output of the operating means provides a digital modulated wave.
The AM modulated wave eliminating circuit according to the present invention extracts the composite wave composed of the AM carrier wave in-phase signal, the AM carrier wave reverse-phase signal and the digital modulated wave from the AM data multiplex modulated wave by the extracting means, presumes the value from the extracted composite wave on the basis of the phase modulated component of the AM stereo modulated wave by the presuming means, multiplies a presumed value on the basis of the phase modulated component of the AM stereo modulated wave by AM carrier wave in-phase component and adds the multiplication output to the composite wave, thereby providing a digital modulated wave.
Furthermore, the AM modulated wave eliminating circuit according to the present invention performs AM modulation of a carrier wave having a frequency fc with an analog signal wave, and eliminates an AM modulated wave from an AM data multiplex modulated wave composed of digital modulated wave signals multiplexed at a location of a frequency (fc+f.alpha. and a location of a frequency (fc-f.alpha.) which are axially symmetrical with regard to a frequency axis of the carrier wave having a frequency fc within a frequency band of the AM modulated wave, characterized in that it comprises A/D converter means which samples the AM data multiplex modulated wave with sampling pulses having a frequency four times as high as that of the carrier wave and performs A/D conversion of sampled signals, a sampling point exchanging circuit which exchanges a discrete value output subjected to the A/D conversion at a sampling point (4m:m=0, 1, 2, 3 . . . ) with a discrete value output subjected to the A/D conversion at a sampling point (4m+3) and exchanges a discrete value output subjected to the A/D conversion at a sampling point (4m+1) with a discrete value output subjected to the A/D conversion at a sampling point (4m+2), and an adder which adds the discrete value output exchanged by the sampling point exchanging circuit to a discrete value output subjected to the A/D conversion by the A/D converter means.
In the AM modulated wave eliminating circuit according to the present invention, the AM data multiplex modulated wave is sampled at the frequency four times as high as the frequency of the carrier wave and the sampled signals are subjected to the A/D conversion by the A/D converter means. When m=0, 1, 2, 3, . . . , the discrete value output subjected to the A/D conversion at the sampling point (4m) is exchanged with the discrete value output subjected to the A/D conversion at the sampling point (4m+3) and the discrete value output subjected to the A/D conversion at the sampling point (4m+1) is exchanged with the discrete value output subjected to the A/D conversion at the sampling point (4m+2), and the exchanged discrete value output is added to the discrete value output subjected to the A/D conversion by the A/D converter means. As a result, only a digital modulated wave is sampled from the addition output with the sampling pulse and provided as a discrete output value subjected to the A/D conversion, whereby an AM modulated wave is eliminated.